By declining to build exchanges, the states would pass the burden and costs of the exchanges to the administration that sought this law. And it is far from clear that the administration could operate the exchanges on its own.
Congress didn’t allocate money for administering federal exchanges, and the law as written seems to prohibit federally run exchanges from providing subsidies to individuals. The administration insists that it can provide those subsidies anyway. But if the courts read the plain words of the statute, then federal exchanges couldn’t really function.
Thus states that refuse to create their own exchanges would effectively be repealing a large part of the law—sparing their citizens from the job-killing employer mandate and from assaults on their religious liberty. In some cases people would even be spared from the individual mandate to buy coverage, since in the absence of exchange subsidies more families would qualify for exemptions from the mandate.